ADAPTIVE ATTRIBUTES OF COWPEA GENOTYPES WITH DELAYED MONOCARPIC LEAF SENESCENCE

In annual grain legumes, monocarpic senescence results in the loss of functional leaf area during pod production, which may limit the availability of fixed C to support seed filling. Genetically delayed leaf senescence (DLS) may enhance the seed yield potential of cowpea [Vigna unguiculata (L.) Walp.] by alleviating this deficiency and by increasing crop longevity. Senescence characteristics, temporal distribution of reproductive activity, and seed yields of contrasting cowpea genotypes were compared in three seasons of field experiments at Riverside, CA, and their carbohydrate reserves were monitored during one season. While senescent genotypes exhibited a typically monomodal reproductive distribution, DLS genotypes consistently produced two sets of pods. During the first podset, DLS genotypes had a lower rate of leaf senescence per unit of seed fill than did senescent genotYPes, but tended to produce slightly lower first-podset seed yields. More residual green leaf area and greater plant viability were maintained in stands of DLS genotypes, which led to production of a substantial second podset. Total seed yields were higher from DLS than from senescent genotypes under full-season conditions. During the first podset, nonstructural carbohydrates were depleted from stems of senescent plants, while starch and sucrose accumulated in stems of DLS plants. The timing and extent of reproductive cutout from the firSt Podset were similar in senescent and DLS genotypes, despite the perennialistic growth pattern of DLS plants. The delayed leaf senescence traits expressed in these experiments may enhance adaptation to variable- or long-season conditions, and to cropping systems in which multiple harvests of mature pods are practiced.